I tend to sanitize at the beginning of the season, but I use this method:
http://rvbasics.com/techtips/sanitiz...er-system.html
The link is no longer valid, but I'll paste the relevant text here:
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A friend and fellow full time RVer, Bill Randolph told me how he uses something other than household bleach which works well for him and I also started using it. I think it is worth passing on to you. Before he retired, Bill spent twenty years in the swimming pool business. He's an expert when it comes to sanitizing swimming pools and spas and says the same rules apply to RV fresh water systems.
Bill uses Chlorinating Concentrate (Sodium Dichloro-s-Triazinetricone or Sodium Dichlor for short). Sodium Dichlor contains 62% available chlorine. Compare that to household bleach which has something close to 3%. One pound of Sodium Dichlor is equal to 8 gallons of bleach! Also, household bleach contains other stuff, including a lot of salt, and that salt and other stuff is what causes the bad taste and why you have to flush the fresh water tank so well.
Bill says it takes only 1 teaspoon of the concentrate per 100 gallons of water to initially sanitize the system. Remember to run water through all the faucets. It's okay to use the full teaspoon even on smaller tanks because you will be flushing the tank before adding the water you intend to drink but it seems wasteful.
Like most of us, Bill travels with a near empty tank to reduce weight. So if he arrives at park where he plans to stay and they have well water, he drops a half teaspoon per 100 gallons of the concentrate into the fill tube and fills his water tank. This insures the system will always be sanitized. No, you do not have to flush again. It's the equivalent to drinking chlorinated city water. If you are filling your tank from a source that is already chlorinated then you don't need to add the concentrate.
That said, if you don't like to drink chlorinated water, don't add the concentrate to the water you intend to drink. Assuming, you fill your water tank from a trusted source you should be safe. Or, you can add the concentrate and then filter the water you drink or cook with.
Truth is I almost always seem to be filling the fresh water tank from a chlorinated source (city water supply) so I seldom need to to use the concentrate. And we do filter our drinking water.
This is not a case where more is better. This stuff is concentrated and it's best to use just what Bill recommends.
Because Sodium Dichlor is so highly concentrated you only need to carry a very small container... buy the smallest container available. And it is dry crystals so there is less chance of a spill. However, because it is so concentrated it is highly corrosive so you do have to be careful how you store it and use it. You should be able to find Sodium Dichlor (Chlorinating Concentrate) at any pool supplies or spa store. Bill says there are several brands to choose from but brand should not be a factor in your choice... it's all the same stuff.
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I have a pool, so I had this stuff on hand anyway. I used ~1/4 tsp or less into the tank, filled it, ran the pump to get the water into the water heater and out all the faucets. Refilled the tank so it's full, and then let it sit a few hours or whatever. Doesn't really matter. Then drain and I'm good to go--no need to refill or rinse. This is my main "de-winterizing" step. It may be overkill but it's just not that big a deal doing it this way, and this way I know I'm starting with a clean tank each year.